1. Field of the Invention
The present invention relates to a system for detecting an electric leakage between a high-voltage direct-current power supply and a body of a vehicle having the power supply.
2. Description of the Related Art
A high-voltage direct-current power supply such as a high-voltage battery or a fuel cell, which is mounted on electric, hybrid, and fuel-cell vehicles or the like, is not usually connected to the earth in order to prevent electric shock. Such a high-voltage direct-current power supply isolated from the earth needs to be monitored on its leakage resistance so as to promptly deal with the electric leakage between the power supply and the earth. There have been proposed various techniques for detecting a leakage resistance.
For example, Japanese Laid-open Patent Application Hei.6-153303 (paragraph [0010]; FIG. 1) discloses a leakage detection apparatus wherein a leakage detection circuit is provided on each of positive and negative electrode sides of a high-voltage direct-current power supply, and a changeover switch is provided between the leakage detection circuits and an earth. The apparatus includes a pair of a protective resistance and a leakage detection resistance, the changeover switch, a voltage measuring unit, and a leakage determination unit. A pair of the protective resistance and the leakage detection resistance, which are connected in series, is coupled to each of the positive and negative electrode sides of the power supply mounted on a vehicle and separated from a body ground of the vehicle. The changeover switch selectively grounds one end of each leakage detection resistance to the body ground. The voltage measuring unit measures a voltage of the power supply separated from the body ground. The leakage determination unit determines the leakage by voltage or current values on the opposed ends of each leakage detection resistance.
Japanese Laid-open Patent Application No. 2002-325302 (paragraph [0015]; FIG. 2) discloses a leakage detection apparatus for an electric vehicle. The leakage detection apparatus includes a first leakage detection circuit, a second leakage detection circuit, and a control circuit. The first leakage detection circuit detects the presence or absence of an electric leakage of a power supply device. The second leakage detection circuit detects a magnitude of a leakage resistance of the power supply device when the leakage is detected. The control circuit switches the second leakage detection circuit into a leakage resistance detection mode when the first leakage detection circuit detects the leakage of the power supply device. In the apparatus, the first leakage detection circuit detects the presence or absence of the leakage of the power supply device. When the leakage is detected, the control circuit switches the second leakage detection circuit into the leakage resistance detection mode, and then the second leakage detection circuit detects a magnitude of the leakage resistance of the power supply device.
The above-mentioned leakage detection apparatuses, however, use a separate leakage detection circuit for detecting the leakage on the positive and negative electrode sides, which results in the complexity of the circuit. Furthermore, in the apparatus disclosed in Japanese Laid-open Patent Application Hei.6-153303, the body ground is used as a reference to carry out the leakage detection. High voltage is applied to a changeover switch, which is an alternative switch, and therefore the switch corresponds to two ON/OFF switches. Because theses leakage detection apparatuses require a plurality of switch elements having high voltage resistance as mentioned above, the apparatuses are undesirable in view of the size and the cost.